Sorry for the bad news but more than likely you have a transmission or internal shifter problem. The clutch would not keep it from shifting although it could make it harder to shift. I once had to ride mine back from out of the woods with no clutch linkage and no way to disengage the clutch and it wasn´t really that bad, I just couldn´t go slow. Do you have anything unusual like large chunks of metal or shavings in the oil?